MOUNTING DIATOM ACEjE. — PALMELLACEjE. 317 



wish to select certain particular forms from a Diatomaceous sedi- 

 ment which has been obtained by the preceding processes, either 

 of the two following modes may be put in practice. A small 

 portion of the sediment being taken up in the Syringe or Dipping 

 tube, and allowed to escape upon the slide, so as to form a long 

 narrow line upon it, this is to be examined with the lowest power 

 with which the object we are in search of can be distinguished 

 (the Erector and Draw-Tube, §§ 63, 64, will here be very useful) ; 

 and when one of the specimens has been found, it may be taken- 

 up, if possible, on the point of the hair, and transferred to a new 

 slide, to which it may be made to adhere by first breathing on the 

 surface. But if it be found impracticable thus to remove the 

 specimens, on account of their minuteness, they may be pushed 

 on one side of the slide on which they are lying ; all the remainder 

 of the sediment which it is not desired to preserve may be washed 

 off ; and the objects may then be pushed back into the middle of 

 the slide, and mounted in any way that may be desired. 



239. Palmellacece. — To the family thus designated belong those 

 two Genera which have been already cited as illustrations of the 

 humblest types of Vegetation (§§ 185, 188) ; and the other forms 

 which are associated with those are scarcely less simple in their 

 essential characters, though sometimes attaining considerable di- 

 mensions. They all grow either on damp surfaces, or in fresh or 

 salt water ; and they may either form (1) a mere powdery layer, of 

 which the component particles have little or no adhesion to each 

 other, or they may present themselves (2) in the condition of an 

 indefinite slimy film, or (3) in that of a tolerably firm and definitely 

 bounded membranous ' frond.' The first of these states we have 

 seen to be characteristic of Palmoglcea and Protococcus ; the new 

 cells, which are originated by the process of binary subdivision, 

 usually separating from each other after a short time ; and even 

 where they remain in cohesion, nothing like a frond or membran- 

 ous expansion being formed. The 'Hed Snow,' which sometimes 

 colours extensive tracts in Arctic or Alpine regions, penetrating 

 even to the depth of several feet, and vegetating actively at a 

 temperature which reduces most plants to a state of torpor, is 

 generally considered to be a species of Protococcus ; but as its celLs 

 are connected by a tolerably firm gelatinous investment, it would 

 rather seem to be a Palmella. The second is the condition of the 

 Genus Palmella; of which one species, the P. cruenta, usually 

 known under the name of ' Gory Dew,' is common on damp walls 

 and in shady places, sometimes extending itself over a considerable 

 area as a tough gelatinous mass, of the colour and general appear- 

 ance of coagulated blood. A characteristic illustration of it is also 

 afforded by the Ecematococcus sanguineus (Fig. 146), which chiefly 

 differs from Palmella in the partial persistence of the walls of the 

 parent-cells, so that the whole mass is subdivided by partitions, 

 which enclose a larger or smaller number of cells originating in the 



